This invention relates generally to control valves, and more particularly, to adjustable valves for controlling the flow of hydraulic fluids. Still more particularly, the invention relates to control valves having a limited range of adjustment and a self locking means to prevent the valve from becoming out of adjustment. The invention has particular application in the hydraulic control circuit of a recloser, an apparatus used in the distribution of electrical power.
In general, a recloser is a circuit breaking or interrupting device used in the distribution of three phase or single phase electrical power. Like a circuit broker, when a fault or other system disturbance is detected on the circuit, the recloser operates to physically separate the current-carrying contacts in each phase of the circuit, thereby opening the circuit and preventing the continued flow of current.
A recloser differs from a circuit brier in that a circuit broker generally opens a circuit and maintains the circuit in the open position for a relatively long period of time, while a recloser operates to automatically open and reclose the circuit several times in quick succession according to a predetermined pattern or sequence. By momentarily opening and then reclosing the circuit, the recloser allows temporary faults repeated chances to clear or be cleared by subordinate protective devices. Should the fault not clear after the recloser has completed its programmed sequence of open and reclose operations, the recloser recognizes the condition as a permanent fault and locks the circuit open. In cooperation with the various other protective devices, the recloser thus has the ability to distinguish between temporary and permanent faults on a circuit. The use of a recloser to reclose a circuit several times often eliminates the need to take a circuit out of service when the fault or disturbance on the line was of a momentary duration only.
A typical recloser includes three major subsystems: (1) a control system for detecting the presence of an overcurrent and for initiating and scheduling the sequence of opening and closing operations; (2) the interrupters, which function to open and close a set of current carrying contacts in each phase of the circuit; and (3) the operating mechanism which provides the energy necessary to open or close the contacts. A mechanical linkage connects the interrupters and the operating mechanism.
The proper operation of an electrical power system includes maintaining the optimal time-current coordination between all the protective devices that are located between the load and the power source. To properly coordinate these devices, a study is made comparing the time it takes the individual devices to operate when specific levels of current pass through the protective devices.
The objective of the coordination study is to determine those characteristics, ratings, and settings for the overcurrent protective devices that will ensure that the minimum unfaulted load is interrupted when the protective devices operate to isolate a fault or overload anywhere in the system. At the same time, the devices and settings selected must provide satisfactory protection against overloads on the equipment, and must interrupt short circuits as rapidly as possible.
Every protective device includes a time-current characteristic trip curve. When making a coordination study, the time-current characteristic curves of all the various protective devices in the circuit are plotted together. A certain time interval is maintained between the curves in order to ensure the correct sequential operation of the devices. These intervals arc required because the protective devices include various speeds of operation and tolerances.
Like other circuit opening and closing devices, the time-current tripping characteristic of the recloser must be carefully coordinated with the other protective relays, fuses, and circuit breakers, both upstream and downstream from the recloser. Such coordination is desirable so that the circuit breakers and protective devices closest to the fault will operate before the more remotely positioned devices. In order to properly coordinate a particular recloser with other protective devices in the system, it is imperative that the recloser have a predictable time-current trip characteristic throughout its entire expected fault current range.
A recloser is typically installed in electrical distribution systems in a main or branch line that supplies subsidiary lines which, in many instances, are protected by fuses. The recloser is generally pre-adjusted to first execute a sequence of one or two fast opening and reclosing operations in the event that a fault occurs downstream of the recloser. During this sequence, many transient faults will clear themselves before the downstream fuse has time to operate. Based on the available fault current, a particular recloser is chosen and adjusted such that the time for opening the circuit during these fast trip operations is faster than the melting time-current characteristic of the fuse, so that the recloser itself is relied upon for attempting to clear the fault without damage to the fuse.
Where the fault does not clear during such a fast opening sequence, the recloser automatically changes its time-current characteristic such that subsequent opening operations are delayed for a time that is sufficient for the downstream fuse to melt and open the faulted circuit. If the downstream fuse melts and thereby clears the fault, or if the fault clears for any other reason during any of the successive operations, the recloser closes and maintains power on the line. If the fault does not clear during the total sequence of fast and delayed opening operations, the recloser opens a final time and automatically locks out the protected line from its source.
A conventional recloser is supplied with a fast or non-delayed time-current characteristic trip curve, and usually at least two delayed or retarded trip curves. Depending upon the coordination requirements dictated by the protective devices upstream and downstream from the recloser, one of the two available delayed characteristic curves is selected before the recloser is placed in service. Once selected, it is important for the customer to be assured that the device will operate in accordance with the selected time-current characteristic curve so that coordination of the protective devices is maintained. As stated above, such coordination ensures that the smallest possible portion of the electrical distribution network will become de-energized when a fault exists so the customers on unfaulted lines on remote parts of the network will be unaffected and will not be inconvenienced.
Although prior art reclosers have been used successfully for many years, the reclosers have not always provided optimum precision and uniformity with respect to their time-current trip characteristics. Variations in these characteristics result from irregularities in manufacturing tolerances and unpredictable dynamics over a widely varying range of currents between the minimum trip current setting and the maximum fault current interrupting rating of the recloser.
The control system in many present day reclosers operates hydraulically. In conventional hydraulically controlled reclosers, the time it takes for the recloser to open the contacts in the interrupters, as well as the time delay for the delayed operations, are controlled by the degree to which the flow of hydraulic fluid in the controller is restricted. Although explained in more detail below, when the delayed trip is desired, the flow of hydraulic fluid is restricted by the sizes of orifices in various ports and valves. The sizes of the orifices are predetermined by design criteria and then manufactured in accordance with that design. Unfortunately, given the manufacturing tolerances of the components which comprise the hydraulic control system, as well as variances in the springs that are employed in various control valves or metering devices, the recloser control system, after manufacture or after extended use in the field, may not provide the precise metering of hydraulic fluid that is necessary to provide the exact time-current characteristic curve desired.
When such reclosers originally became available and were placed in service, they included no way to adjust or alter the flow of hydraulic fluid to compensate for varying manufacturing tolerances or changes to the hydraulic system that occurred over time. In the more recent past, manufacturers have produced devices intended to provide the adjustment previously lacking. The devices, generally known as timing blocks, were "add-on" devices which replaced original components of the recloser. The timing blocks included bores which were aligned with exit ports or orifices in the recloser hydraulic control system. The flow of hydraulic fluid through these bores was controlled by the use of spring-loaded adjustment screws. As designed, the timing blocks permitted incremental adjustments so as to increase or decrease the size of the conduit through which the hydraulic fluid passed in order to more precisely meter the flow. Theoretically, such designs would permit adjusting the flow of hydraulic fluid so as to yield the desired time-current characteristic curve.
The theory of using adjustable timing blocks had merit and the blocks themselves have been employed with varying measures of success. Nevertheless, significant disadvantages and liabilities exist with their use.
First, the timing blocks were relatively large. Once installed in the recloser, the blocks protruded so far as to significantly decrease the dielectric clearances that are required for proper and safe operation of the recloser. This decrease in clearance greatly increased the likelihood of flashovers occurring within the tank of the recloser. Also, the heads of the adjustment screws protruded even farther beyond the surface of the timing block. This even further reduced the necessary clearances.
The timing blocks also suffered from the disadvantage that they included no means for preventing gross misadjustment. For example, it was not uncommon for the timing screws to be adjusted in the field to such an extent that there resulted a severe miscoordination with the other protective devices. Even where the screws were originally adjusted properly, it was found that the spring loaded screws frequently loosened over time due to the inherent vibration created by the flow of 60 Hz current through the apparatus. This could also lead to a loss of coordination.
Additionally, to properly adjust the trip times, in many instances, the screws had to be loosened to such an extent that they were barely engaging the threaded bore of the timing block. Due to improper handling or the vibration previously described, the screws could become unseated and fall out of their bores in the timing block. This would cause a shift in the time-current characteristic curve and possibly a loss in coordination with the other protective devices. The displaced screw and spring could also become lodged within the interrupter mechanism and create short circuits within the recloser or physically prevent the operating mechanism from opening or closing the contacts when required.
Accordingly, despite the long and successful use of hydraulically controlled reclosers, there remains a need in the art for a reliable means for adjusting the flows of hydraulic fluid to compensate for slight manufacturing variances, as well as to adjust the recloser to continue to meet the desired time-current characteristic curve as the characteristics of springs and other components of the control system change over time after extended use in the field. The adjustment means should provide a simple and effective method of adjusting the time-current characteristic curves, both at the factory and under field conditions. The apparatus should provide this adjustment without reducing the dielectric clearances that are necessary for safe and effective operation of the recloser, and should include a means for preventing the apparatus from shifting or falling out of adjustment during shipment, installation, or use. Additionally, the apparatus should be immune to vibration. It would be especially desirable if the device could be installed in the many thousands of existing reclosers now in use. Finally, it would be ideal if a means were provided for limiting the degree of adjustment so as to prevent excessive or gross misadjustment of the apparatus.